During October 1982, surveys were carried out to assess the
impacts of opencast mining on rivers and coastal systems in New
Caledonia. Features shown on vertical air photographs taken by
the Institut Géographique National in 1954-1955 (scale 1
:40,000) were compared with those on air photographs taken in
1976 (scale 1 :20,000), and with the present configuration
examined in the field. In northern New Caledonia in 1982 many of
the rivers still showed the effects of torrential runoff during
Cyclone Gyan (24-25 December 1981), when large quantities of
sediment of varying calibre were carried downstream. Severe
flooding was recorded for the Diahot, Koumac, and louanga rivers
in the north-west, and there was torrential flow down ungauged
streams to the west coast between Voh and Koumac.

The problem of deciding how far opencast mining has influenced
the form and sediment content of river channels and to what
extent it has modified features at river mouths and in adjacent
coastal and nearshore environments was considered on the basis of
each of the major river systems draining to New Caledonia's
western and eastern coastlines (fig. 2). These assessments are
given below, taking the river systems in counterclockwise order
from the north.

Diahot

Opening north-westward into the Baie d'Harcourt, the Diahot
(catchment: 620 sq km) carries a substantial load of fine-grained
sediment, mainly sand and silt, derived from the Mesozoic
sedimentary formations of the Central Ranges and from the
mica-schists of the Panié massif. Sediment delivered to the
river mouth, mainly during flood episodes, has been built into
channelside levees prolonged as sedimentary jetties forming the
framework of a delta that includes extensive mangrove swamps as
well as dry saline plains. The rate of sediment yield may have
been increased in historical times by the cutting and burning of
forest and woodland vegetation and the introduction of grazing,
resulting in accelerated soil erosion on friable materials.

Néhoué

This river system drains a basin of 229 sq km in north-western
New Caledonia, its northern tributaries flowing from the
schistose highlands of the Forêt d'Ougne Range, while the
southern tributaries include streams flowing from the ultrabasic
massif of Tiébaghi. This massif has been mined for chromium ore,
at first opencast, then mainly in shafts and edits. The Néhoué
and its tributaries have water stained red with ferruginous clay
in suspension, and it is likely that their loads have been
augmented by mining waste and material eroded from areas where
the surface has been disturbed by prospecting. However, although
some red mud has reached the deltaic region, features at the
mouth of the Néhoué have so far been little modified by the
effects of mining in the catchment.

Koumac

The Koumac and its tributaries drain an extensive catchment
(area: 258 sq km) in northern New Caledonia, where the geological
formations include schists, sandstones, limestones, and phthanite
chert, a form of hard cryptocrystalline silicified sediment. This
latter material has broken up into coarse gravels, present in the
Pleistocene river terraces and piedmont fans bordering the upper
valleys, and abundant in the river channel downstream past Koumac
to the delta region. During episodes of flooding, gravel has been
carried to the mouth of the river, and longshore drifting has
thence swept it northwards along the coastline to form beaches,
culminating in a long recurved spit at Tangadiou. Although there
has been mineral prospecting within this catchment, it has not
been affected by opencast mining, and the presence of gravel in
the river channel and along the coastline north of the delta
cannot be attributed to mining impact. It is thus evident that if
a source of coarse-grained sediment (in this case Pleistocene
terrace and fan gravels) is naturally available within such a
river catchment, such material can be carried down to the coast,
notably during episodes of floodwater discharge that occur during
tropical cyclones. Delivery of coarse sediment loads to river
mouths is thus not in itself a reliable indication of the impact
of opencast mining in New Caledonia.

Nevertheless, it appears that the addition of gravelly beaches
to the coastline at and north of the Koumac delta is a very
recent phenomenon, for the older parts of the delta are dominated
by sediments no coarser than sand. A probable explanation is that
slope wastage, and especially the erosion of Pleistocene river
terraces and piedmont fans, results from increased and
accelerated runoff due to the reduction and impoverishment of the
vegetation cover, mainly forests, within

this catchment by bushfires. The Melanesian people used fire
extensively in this region, and bushfires have occurred
frequently in the era of European occupation. If this is the
case, the delivery of a gravelly load to the coast by the Koumac
during floods is indeed an anthropogenic impact, even though it
is not due to opencast mining.

Oué Bouameu

South of Koumac the ultrabasic Kaala massif is fronted by a
broad piedmont fan composed of a variety of coarse to fine
sediments washed down from the steep slopes during Pleistocene
times. In the Holocene this has become a relatively stable
landform, crossed by a series of parallel small watercourses, but
one of these, the Oué Bouameu (catchment: 13 sq km), has become
larger and deeper than the others, and is carrying a load ranging
up to cobbles and small boulders, much of which has been derived
from spilling and slumping waste fans around the opencast nickel
mines on Mount Kaala. The Oué Bouameu has taken a variety of
courses across a lobate fan at Karembé (fig. 13), where
recurrent flooding of a farmstead has prompted the cutting of an
artificial channel to deflect the river southwards. During
Cyclone Cyan, when over 430 mm of rain fell within a period of 24
hours at Koumac, the Oué Bouameu flowed torrentially and carried
large quantities of sediment ranging from fine red clay up to
cobbles 12 cm in diameter downstream to the coast (plate 6),
where the coarser sand and gravel have accumulated as a beach in
front of a shoreline that was previously mangrove-fringed and
fronted by tidal mudflats (plate 7).

This beach has grown northwards (plate 8), past the Karembé
wharf, where a rock breakwater is under construction to the north
to intercept the drifting beach material and prevent it spreading
to the Koumac shore. Wave action is thus redistributing material
that was brought down to the coast in the last flood. Near the
southern end of the deposited sand and gravel, southerly waves
have built a cuspate spit, which is being driven back into
mangrove-covered clay flats in the form of a chenier.

Red clay washed down from the hilltop mine has been deposited
in the nearshore zone, and the sea is stained red-brown by this
sediment in suspension. Local people believe that this pollution
has impeded coral growth in nearshore waters, and that the
inshore fishery has also been impoverished, both in terms of
quality and quantity of fish available. But apart from structural
damage during the flood (plate 9), the mangrove areas have been
little affected and mangrove growth is continuing on red mud
accretion. Construction of barrages and sediment traps around the
opencast mine is now reducing the sediment yield, but substantial
masses of waste material have already spilled and slumped down
slopes in the headwaters of the Oué Bouameu, and much of this
will be carried down to the coast during future episodes of
flooding.

louanga

Like the Koumac, the louanga (catchment: 522 sq km) drains an
area of varied geology, mainly schists, sandstones, and phthanite
cherts. it has so far been little affected by the opencast nickel
mining on Mount Kaala and on the Massif d'OuazangouTaom; its
channel load at Kaala-Gomen is a mixture of sand and gravel, the
latter mainly of chart. Some of this material reaches the delta
region, where ridges of fluvial sand and gravel, with some shelly
and coralline material, have been built up as fringing beaches or
cheniers emplaced on clay plains. However, the seaward fringes of
this delta are mainly mangrove swamp, and gravel beaches and
spits of the kind seen north of the Koumac delta have not yet
developed. In the past few years there have been changes in the
meandering channel downstream from the highway, associated with
aggradation and bank erosion, but this does not seem to be a
direct consequence of anthropic factors.

Tinip

The Tinip stream (fig.14) drains a small catchment (area: 12
sq km) on the western slopes of Mount Ouazangou. Like the Oué
Bouameu, it has been strongly affected by opencast nickel mining
in its headwaters, the stream channel having been widened to
accommodate an increased and coarsened sediment yield derived
from the waste products of mining. An air photograph taken in
September 1976 shows a large mass of mining waste in the valley
on the western slopes of Mount Ouazango (see fig.14), and this
has subsequently moved down the river. In October 1980 the Tinip
had an aggraded channel at the highway crossing (plate 10),
mining waste having reached this sector, but by October 1982 much
of this material had been sluiced down-stream, re-exposing the
trunks of trees that had been killed and buried by the earlier
aggradation (plate 11). This change implies some success in
attempts to control the flow of overburden discharged from the
hilltop mine, the last major floods (Cyclone Freda in March 1981,
then Cyclone Gyan in December) having driven channel fill down
the river where the channel has widened and aggraded (plate 12).
In the Baie de Gomen a delta of red mud, sand, and fine gravel is
developing. The inshore waters of the Baie de Gomen are stained
red with suspended clay, and the prop-roots of Rhizophora are
also coated with red clay, although this has not prevented their
continued growth. A cloud of suspended clay extending south from
the river mouth is visible on an air photograph taken in 1976,
but is absent from a 1954 air photograph of the same area, taken
before the periods (1955-1964 and 1967-1977) of intensive mining
on Mount Ouazangou.

The Tinip is thus an example of a stream that has been
modified along the whole of its length by the inflow of mining
waste materials from its headwater regions. The coastline at its
mouth has been modified by the formation of an "anthropic
delta," and the nearshore waters show the effects of red
clay pollution, which has not proved adverse to mangrove growth
but may well have depleted local fish populations.

Like the Tinip, the Ouaco (catchment area: 35 sq km) has been
much modified by the effects of opencast mining on Mount
Ouazangou. Its sediment yield has been increased and its deltaic
shoreline is prograding as the result of the additional sediment
supplied to the river mouth. The nearshore sea water here is
generally discoloured by red clay washed down the river and
retained in suspension.

Poué Koué

Draining the southern slopes of Mount Ouazangou, the Poué
Koué (basin area: 32 sq km) has also been greatly affected by
the opencast hilltop mining. In fact, the mining waste is
received from three small peridotite massifs in the headwaters
(Ouala, Tsiba, and Oualéat), which have been intensively mined.
Being only four to six km inland, their wastes have quickly
reached the river mouth. The river channel is overloaded with a
mixture of sediment ranging from red clay to small boulders, and
the lobate piedmont fan that formed this part of the coastline is
being prograded by the addition of this material to form a delta
around the river mouth. Extensive red staining has polluted the
coastal waters off the Poué Koué.

Taom

Draining the southern slopes of the Massif d'Ouazangou-Taom,
this river (basin area: 105 sq km) has been less affected by the
inflow of mining waste, which has not spread as far downstream as
in the Poué Koué. Mining has occurred at only three sites 15 to
20 km upstream, so that the effects have been less dramatic than
in the smaller Tinip and Poué Koué basins. At the highway
bridge the watercourse is slightly incised and gravelly, but the
sediment yield to the mouth of the river has been small and the
effects of mining activities on coastal and nearshore
environments have so far been only slight.

Oué Pouanlotch

This river drains a deep gorge between Mount Taom and Mount
Témala. The upper part of its catchment (area: 92 sq km) has not
been affected by hilltop nickel mining, but the summit area and
the eastern slopes of Mount Taom have been intensively
prospected, leaving a dense network of trackways (visible on 1976
air photographs). Below Boyen the river flows across a
Pleistocene alluvial plain, and although its channel load at the
highway crossing contains peridotite boulders from the massif,
the augmentation of sediment yield to the coast has so far been
meagre and the effects of mining activities remain very slight.

Témala

Although nickel mining has taken place in the coastal Mount
Katépahié and Massif de Voh regions, the sediment yield from
the Témala catchment (352 sq km), like that from the adjacent
Oué Pouanlotch, has been more affected by the soil erosion that
has resulted from the grazing of pastures on basaltic hill
country in the relatively dry environment around Témala (Iltis
1979). Occasional heavy rains have washed substantial quantities
of grey clay and silt from these eroding hillsides into the
Témala River, and downstream to the extensive marshes and
mangrove swamps on its delta plains west of Voh. At the highway
bridge the river is clear, with shoals of grey sand and gravel,
and the impact of the opencast mining activities here and at the
river mouth has also been slight.

Voh

The Voh River (catchment area: 227 sq km) drains an extensive
upland region dominated by the large ultrabasic Massif de
Koniambo, the north-western part of which is called the Massif de
Voh. Hilltop mining was intensive here from 1900 to 1940, when
the region was the leading producer of nickel in New Caledonia,
exceeding even the Thio area. Hillsides remain scarred by this
mining activity and there is no doubt that the Voh and its
tributaries had augmented sediment loads during the mining phase,
but the inflow of mining waste has diminished, and the river
channel, which was probably widened as the augmented sediment
yield passed through, now appears relatively stable. It is likely
that the delta at the mouth of the Voh has been prograded as the
result of augmented sediment discharge, but these effects have
diminished and the present mangrovefringed coastline is no longer
strongly affected by these historical mining activities in the
hinterland.

Massif de Koniambo

This ultrabasic massif, between Voh and Koné, is one of the
larger areas that has so far not been mined extensively for
nickel; it is regarded as a reserve for future nickel production
and will be exploited in due course. Meanwhile the summits and
slopes are almost intact, their weathered lateritic mantle held
in place beneath maquis scrub, and the Piedmont apron on their
seaward flank shows a number of narrow, gravelly watercourses
(e.g., Le Pandanus, La Confiance) that have not been modified by
mining activities. Their present dimensions and sediment loads
are thus contrasted with those of the Oué Bouameu and Tinip
streams, which were once like this.

Koné and Pouembout

Ultrabasic terrains at the eastern end of the Massif de
Koniambo form only a small proportion of the catchment (area: 272
sq km) of the Koné and its tributaries, and at the highway
bridge the river carries a mainly sandy load downstream to a
mangrove-fringed delta that is not prograding as rapidly as the
deltas of rivers strongly affected by mining activities. The same
applies to the adjacent Pouembout river system (catchment basin:
339 sq km). However, the left bank tributaries of the Pouembout
are influenced by inflow of graveily peridotite from former mines
on the crest of the Massif de Kopéto, notably during floods that
accompany cyclones. This coarse material extends down to the
confluences with the Pouembout, finer sediment having been washed
further downstream.

Tiaoué

The Tiaoué has a relatively small catchment (66 sq km) on the
western side of the Massif de KopetoBoulinda. Although there has
been some prospecting and hilltop mining on the upper fringe of
this catchment, the river system has been little affected. It
carries a sparse load of grey gravel, sand, and silt, largely
derived from the erosion of basaltic hill country, and has not
yet built a delta.

Népoui

The Népoui, and its tributary, the Rivière Salée (combined
catchment area: 177 sq km), drain deeply dissected ultrabasic
massifs where opencast nickel mining has been extensive (fig.
15). In the 1970s "boom" period this was the most
productive mining region in New Caledonia, but it has been
overtaken by Kouaoua, and in fact production ceased from the
Massif de Kopéto on I January 1983. As a result of mining, vast
masses of overburden have flowed and slumped down the slopes
bordering hilltop mines in the Massif de Kopéto-Boulinda (and
especially in the Massif de Graunda-Boulinda), and the river
channels of the Népoui (as of the neighbouring Ouha to the
south-east) have been greatly modified, widening to accommodate
large quantities of sediment ranging from red clay through sand
and gravel to cobbles and boulders. Figure 16 shows details of
the impact of this opencast mining on the geomorphology of Mount
Graunda, between the Népoui and Ouha valleys. Mining has here
removed much of the summit plateau, and hillside waste fans form
depositional cones, one of which has partly blocked the Ouha
valley, forming a ponded river. The channels of the Népoui and
Ouha have become wide and gravelly, the streams flowing through
braided channels. Plates 2, 3, and 13 show the features produced
by the inflow of mining waste to the Népoui, in contrast with
the unmodified river channel upstream (plate 14).

The Népoui River flows into an elongated embayment (fig. 17)
alongside the limestone Presqu'île de Népoui, and at its mouth
mangroves are extending rapidly on to broad red mudflats (plate
15) produced by the deposition of iron-stained clay washed down
from the disturbed lateritic overburden in the hilltop mines.
Comparison of air photographs taken in 1954 and 1976 show that
the mangrove fringe advanced seaward by up to 400 metres on the
shores of the Népoui delta in this period, and this advance is
continuing. The waters of the Baie de Népoui have been shallowed
by this sedimentation and red clay staining is extensive.
Successive floods are washing coarser sediment down the Népoui.
In addition to red clay, some sandy material has arrived at the
river mouth, pebble gravels have extended to the lower reaches,
and eventually coarser gravel will arrive at the coast. Already
severely affected by mining activities, the Népoui. delta region
is destined to receive much more sediment of gradually increasing
calibre. Mangrove growth has not been impeded, and has probably
been stimulated by the accretion of red mud, but as coarser
sediment reaches the delta area, it is likely that the mangrove
cover will be disrupted.

As mentioned already, the Ouha (catchment: 84 sq km) has also
been affected by opencast mining on the crests of the Massif de
Graunda-Boulinda. At the highway crossing it carries a sandy load
stained by red clay, which is being delivered to its mouth. The
sea is stained red in the Baie de Muéo off the growing delta of
the Ouha. However, the advance of this delta is clearly less
rapid than that of the Népoui. Instead of a general spreading of
mangroves on to accreting mudflats, the advance is here by way of
scattered mangrove establishment.

Baie de la Poya

The Poya and its tributaries (catchment: 418 sq km) drain a
broad lowland flanked by the Massif du Boulinda to the north and
the Massif du Me Maoya to the east. Both areas have undergone
extensive hilltop nickel mining, and the upper tributaries have
received sediment yields augmented by mining waste; but the Poya
is so far little affected. By contrast, the Muonio to the
north-west flows directly from the southern slopes of the Massif
du Boulinda into the broad, shallow Baie de la Poya, and has an
aggraded channel of coarse material stained by red clay. The
Moindah River (catchment: 165 sq km), draining directly from the
mined Me Maoya massif to the west, is also delivering red-stained
silt and clay derived from mining waste into the Baie de la Poya.
Coarser material (pebbles and boulders of peridotite) has been
transported five to six km down from the massif along a deep
gorge, but is deposited well upstream from the river mouth.

Backed by the Me Maoya massif, the catchment of the Rivière
du Cap (area: 174 sq km) is the largest of several river systems
draining to the coast between Poya and Bourail. Its right bank
tributaries have built broad piedmont fans and terraces, while
the main course flows alongside the siliceous sandstones and
cherts of the Montagnes Blanches to enter the sea by way of a
small delta built into an embayment. Downstream the channel load
includes peridotitic gravel and cobbles supplied by the right
bank tributaries. The lower watercourse and the delta receive red
lateritic clay derived from former opencast nickel mining on the
southern slopes of Mount Djiaouma. The river has thus been
moderately affected by mining activities.

Néra

The Néra and its tributaries form one of the larger river
systems on the west coast (catchment: 546 sq km), uniting near
Bourail to enter the Baie de Gouaro by way of a deltaic plain
fringed by sandy beach ridges. Much of the catchment consists of
hilly terrain on basalts and sedimentary rocks, but ultrabasic
massifs occur on the higher inner margins, which form the eastern
edge of the Massif du Me Maoya thrust outliers.

Abandoned cultivation terraces (tarodières) are extensive in
this and other catchments on the central west coast (plate 16),
indicating a precolonial phase of land management by indigenous
people, who may have perceived that their utilization of steep
slopes was resulting in soil erosion; they imply a response to an
earlier phase of anthropogenic erosion on slopes that had been
cleared and cultivated, but the details of this have not been
researched.

Of the various Néra tributaries, the most affected by mining
is the Téné, a tributary of the Boguen, which has its
headwaters in the Massif de Téné, but the effects of this do
not yet extend far downstream and the Néra estuary is a clear
river with shoals of grey silty sand and some gravel. Sandy
material washed into the Baie de Gouaro has been built into
beaches and beach ridges by ocean swell that enters by way of a
gap in the outlying coral reefs.

Nessadiou

The Nessadiou (catchment: 87 sq km) follows a meandering
course westward from the hill country north of Moindou to reach
the sea by way of a small bay-head delta south of Bourail. Its
catchment consists mainly of Mesozoic and Eocene volcanic and
sedimentary formations, and its yield is a mixture of silt, sand,
and fine gravel. It is an example of a river system unaffected by
mining activities, but with a sediment yield probably somewhat
augmented by the clearing and burning of vegetation on hilly
country in its upper catchment. Here, as elsewhere on the west
coast, recurrent burning to maintain pastures has converted
extensive areas of sclerophyllous forest into grassy savannas
with areas of niaouli scrub, formerly confined to low-lying
areas. This weakened vegetation cover results in accelerated
runoff and soil erosion.

Moindou

The Moindou (catchment: 125 sq km) is another basin without
ultrabasic formations and opencast mining. There has been
underground mining of coal in the Moindou region, but this has
had a negligible impact on river sediments. The Moindou carries
mainly clay, silt, and sand down to its marshy mangrove-fringed
bay-head delta.

La Foa

This extensive river system (catchment: 438 sq km) has
headwaters in Palaeozoic and Mesozoic volcanic and sedimentary
formations. Some serpentinite outcrops also occur, but have not
yielded exploitable minerals. La Foa has not been affected by
mining activities. In its lower reaches it carries mainly clay,
silt, and sand into a winding estuary opening into the Baie de
Téremba.

Ouaméni

This river system drains hilly and mountainous country
(catchment: 175 sq km) north-west of Bouloupari, including the
western slope of an ultrabasic massif, Mont Do, where hilltop
mining has not taken place. The Ouaméni delivers a load of
fine-grained sediment, no coarser than sand, to its delta in the
northern part of the Baie de St. Vincent.

Ouenghi

Opencast hilltop nickel mining has been carried out on the
western end of the peridotitic Grand Massif du Sud on the margins
of the Ouenghi catchment (area: 270 sq km), and the sediment load
of this river has been augmented by red clay and by gravelly
material derived from mining waste. These effects have been
reduced, however, by systematic mechanical settling of colloidal
iron hydroxides and by the building of large sediment-trapping
boulder dams around the opencast mines of the Massif de
Kongouhaou, and the impact of mining activities in the lower
reaches of the river, on the Ouenghi delta, and in the adjacent
Baie de St. Vincent have been slight. Nevertheless, recent
attempts to introduce oyster farming in the eastern part of this
bay and the building of shrimp ponds in the Baie de Kuara,
adjacent to the Ouenghi delta (fig. 18), are activities that are
periodically impeded by red-clay pollution from recurrent river
flooding.

Tontouta

Like the Ouenghi, the Tontouta and its tributaries (catchment:
476 sq km) drain a peridotite hinter-I des Roussettes, North of
Bourail, Date and where opencast mining has been extensive on
mountain crests, and on hillslopes, some only few kilometres from
the coast. The river carries extensive gravelly shoals as well as
sand and finer material, and has shallowed nearshore areas round
its mouth in the southern part of the Baie de St. Vincent. The
impact of mining activities as been stronger than in the Ouenghi
catchment because of the lack of conservation works and ecause of
the intensity of mining and waste generation in the Tontouta
catchment in the last 20 years. The sediment yield has increased
at the river mouth, where mangroves have become ore extensive,
spreading back on to the delta below Mont N'dui, as seen when
1954 and 1976 air photographs are compared. Dombéa dome
small-scale localized nickel mining has ocurred in the headwater
regions of the Dumbéa River (catchment: 233 sq km), but the
effects on river loads have been slight; as in other drainage
basins near Nouméa; mineral prospecting has been forbidden in
order to maintain water quality. The sediment yield at the river
mouth, where mangrove and marsh-covered delta (fig. 19) is
rowing, consists mainly of clay, silt, and sand, and the effects
of mining activities have been only slight. Some mangroves
alongside the new highway have been killed by waterlogging due to
road construction, rather than by changes resulting from
hinterland mining.

With a catchment (area: 92 sq km) almost entirely in the
ultrabasic Grand Massif du Sud, La Coulée carries sands and
gravels stained by red clay, a sediment yield augmented by
hilltop mining activities which continued into the 1970s. The
river is extending its delta into the southern part of the Baie
de Boulari, where tidal mudflats are being deposited in a
red-stained sea. Similar features are seen on a smaller scale
along the Plum stream to the east of Mont Dore, where the
hillsides are scarred by gullies and landslides in the weathered
red lateritic mantle.

Rivière des Pirogues

This river (catchment: 152 sq km) has its headstreams adjacent
to the Upper Yaté basin, a sparsely vegetated region of
peridotitic karst, in the vicinity of Ouénarou, where river
piracy is in progress. The headwater region shows features formed
by differential erosion of gabbros and granodiorites associated
with the peridotite Massif du Sud, and has not been mined. The
lower part of the catchment has been much affected by opencast
mining, and by gullying and slumping of red ferruginous lateritic
mantles on valley-side slopes. More than 60 years ago there were
reports of rapid erosion as a sequel to mining in this area
(Compton 1917). The Rivière des Pirogues has a load of sand,
gravel, and cobbles stained by red clay, which is rapidly filling
its estuary in the lee of a sand spit built at the river mouth.
The adjacent sea, shoaly with down-washed sediment, is
red-stained. Similar features occur in the small Rivière Ngo
catchment to the south, opening into Baie Ngo (plate 17).

The branching ria of the Baie de Prony receives a number of
small streams, including the Rivière Bleue and the Ruisseau de
la Bergerie, which carry increased loads of red clay as a sequel
to opencast mining between 1955 and 1968 of the ferruginous crust
of the laterite mantle as an iron ore. Extensive areas north-west
of the Baie de Prony have been denuded of vegetation and soil by
this extraction, and persist as a red clay desert, within which
gully erosion is developing. The streams are washing red clay
down to stain beaches and nearshore waters in the small
embayments (e.g., Baie de la Somme) which branch from the Baie de
Prony. Divers have reported unusually large madrepore species
growing in turbid waters here, indicating that some corals may be
able to adapt to the changed environment. The deeper waters
appear to have been little affected, and coral growth remains
vigorous offshore.

Koué

Draining southward from the escarpment bordering the Plaine
des Lacs, this river (catchment: 150 sq km) has some headstreams
in the area affected by opencast iron ore mining, but does not
yet show significant modification of its channel, which opens
into a winding ria.

Kuébéni

Typical of streams descending from headwaters in the
peridotitic karst region of the Plaine des Lacs, the Kuébéni
(catchment: 38 sq km) carries a load of red clay and pisolitic
ironstone sand, much of which is being deposited in the small
inlet at its mouth, in the lee of the emerged reef islands of Nou
and Néaé. Opencast iron mining in the Goro region between 1937
and 1941 and the subsequent bulldozing of superficial deposits
have increased this sediment load and accelerated deposition at
the river mouth. Systematic prospecting in the 1970s in this
southern part of New Caledonia revealed extensive reserves of
low-grade nickel ore in the superficial formations, and it is
possible that large-scale mining will proceed here in the future.

Yaté.

The original Yaté dam was completed in 1927 and has been
replaced by the modern barrage, built between 1955 and 1958. As a
result, the Yaté (catchment: 450 sq km) has delivered little
sediment to the coast. The extensive, branching reservoir acts as
a sediment trap, receiving red clay and pisolitic sands from the
bordering slopes, a supply that has been augmented by nickel and
some chromium mining, road-making, prospecting, and the depletion
of vegetation by recurrent bushfires.

Pourina

This basin (catchment area: 98 sq km) occupies the southern
part of the eastern slopes of the Grand Massif du Sud; the
watershed rises about 1,000 metres above sea level and slopes are
steep. Erosion is a dominant process, especially in the middle
and lower catchments, where the forest cover thins out. Mining
activity has been limited to nickel prospecting and local
exploitation of chromium. At the river mouth a small sandy
barrier nestles at the head of an embayment and has been slightly
enlarged by the accumulation of ferruginous and chromitic sands
during the past 20 years, without much effect on the outlines of
the embayment.

Ouinné

This catchment (146 sq km) is even more mountainous than that
of the Pourina. The river mouth consists of a small delta built
into a lagoon behind a large sand barrier on which the mining
settlement of Ouinné stands. The configuration of the delta has
been modified by the cutting of channels to allow barges to load
mineral ore close to the mines. Nickel-bearing formations occur
between 200 and 300 metres above sea level on hillsides
overlooking the delta and were mined in the late 1960s. Air
photographs taken in 1976 show that the mouth of the Ouinné had
not yet been affected by augmented sediment supply, but the small
Ué River to the south, draining coastal hills where mining
occurred, had been almost blocked by the deposition of waste
material. It is feared that the effects of nickel mining will
increase here during the next few years in spite of the reduction
in mining activity and the building of dykes to partly enclose
the river mouth.

Kouakoué

This very mountainous small basin (catchment: 77 sq km) is
dominated by the ridge of the Massif de Kouakoué, which rises to
1,501 metres above sea level. It has been subject to exploration
but not to mineral exploitation. The lower part of the valley
widens into a delta. Although the basin has not been mined,
sedimentation appears to have increased at the river mouth, where
a sand spit built of fluvial sediment reworked by wave action
grew substantially between 1954 and 1976.

Ni

Mineral prospecting has occurred within the basin of the Ni
(catchment: 173 sq km), and construction of exploration roads has
strongly increased sediment yield. Comparison of air photographs
taken in 1954 and 1976 shows sand accretion in the delta, where
the river splits into three distributaries. Two of the three
outlets are now partially obstructed by a large fluvial sand and
gravel barrier beach built during this period.

Ngoye

Nickel mining has occurred at several places within this
93-square-kilometre catchment, but the effects on the river and
its delta have so far been slight. The delta, which (like the
Kouaoua delta) includes successively formed beach ridges, is
bordered by a littoral spit, which grew between 1954 and 1976 to
reduce the outflow channel to the sea to a narrow pass.

Comboui

Dominated by the high ranges of the Humboldt massif, this
basin (catchment: 180 sq km) has also been subject to mineral
prospecting, which has greatly increased fluvial sediment yields.
Large quantities of fine-grained sediment have also been derived
from serpentinite hills up to 300 metres high in the lower
valley, where ravine dissection has been in progress since before
the prospecting era, and perhaps from pre-colonial times.
Nourished by these inputs, the Comboui has a widened channel
leading to a substantial delta, with several distributaries, one
of which has been sealed off and two others narrowed as a result
of growth during the last 20 years of sand spits built of
wave-worked fluvial sediment.

Nimbo (Mba)

In this small basin (85 sq km) there has been only local
nickel mining and some prospecting. These activities do not
appear to have greatly affected the form of the river channel or
its mouth, where a small littoral spit built of wave-worked
fluvial sand has been driven landwards in recent years.

To N'deu

In the northern part of this basin (catchment: 72 sq km) there
were two nickel mines at a low level about 3 km inland, worked
intensively at the beginning of the century. Later a chromium
mine was established, and nickel prospecting extended southwards.
Sand and gravel discharged from the mines and areas of
disturbance by prospectors aggraded the river channel and led to
accretion on the delta, but these effects have diminished during
the last few decades. Between 1954 and 1976 there were only minor
changes on the deltaic shoreline of the To N'deu.

Thio

The Thio and its tributaries drain a region of 404 sq km,
dominated partly by the ultrabasic uplands of the Grand Massif du
Sud and partly by the volcanic and sedimentary formations of the
Central Chain. Opencast hilltop mining has been extensive,
especially on the slopes of Mount Ningua, and at the Mines Bornet
above the Nakalé and Nembrou tributaries, where the river loads
have undoubtedly been increased and coarsened as the result of
these activities (plate 18). The most extensive mines, however,
are in the coastal region, notably on the Thio Plateau,
immediately west of the Thio township, where large-scale opencast
nickel mining has been in progress for 80 years (fig. 20). The
Thio Plateau contains the largest nickel deposit in the world. It
has been mined continuously since 1901, and in eight decades has
yielded almost 20 million tonnes of ore, producing about 450,000
tonnes of nickel and cobalt. In 1981 the Thio region was the
second most productive mining centre (the first being Kouaoua) in
New Caledonia.

The outcome has been massive generation of mining waste.
Substantial masses of sediment have slumped down the hillside and
have been washed into the Thio River. It is believed that this
river was formerly navigable by canoe upstream to Nakalé and by
larger boats to a point near Les Pétroglyphes, about four km
from the mouth, but infilling by sediment derived from mining
waste has been rapid and such navigation is no longer possible.
Below St. Pierre-Nakalé the impact of mining waste flowing into
the Thio channel has been obvious, especially since the 1960s,
although some changes had been observed twenty years previously,
when mechanization of mining greatly increased the rate of waste
production and downslope movement of discharged debris. Now the
banks and bedload of the Lower Thio consist entirely of sand and
gravel washed down from the Thio Plateau. An alluvial island 400
metres long has been completely blanketed by this debris, its
scrubby vegetation cover destroyed.

The Thio delta, developed on either side of the rocky ridge of
Bota Mere, has shown many changes in configuration on photographs
taken in recent decades, and its present outlines differ
substantially from those shown on the 1976 air photographs (plate
19). Each flood brings additional sediment, ranging up to sand
and gravel, down to the river mouth, where it is sorted and built
into spits and bars by wave action. Successive ground and air
photographs indicate progradation relative to Bota Mere, with the
formation, growth and widening of spits, the development of
deltaic islets, the silting of distributary channels, and the
infilling of small lagoons permitting mangrove establishment.
Mining activities have thus accelerated delta development and
augmented the sand supply to the beaches that extend northward
from the mouth of the Thio. In recent years, rock dams and
interception basins have been built to reduce the amount of
sediment yield from the mining areas.

Dothio

The Dothio (catchment: 77 sq km) drains a small basin
north-west of the Thio that has a head-water region of volcanic
and sedimentary rocks and several ultrabasic massifs near the
coast where hilltop mining is active or has been carried out in
recent decades. The slopes of the Thio Plateau have several areas
where mining waste has spilled down and some large landslides. An
attempt has been made to control the discharge of coarse waste
material down the valley of the Wellington, tributary to the
Dothio, by bulldozing boulder levees and a diversion dam (plates
20, 21, 22), but floodwaters will continue to deliver some of the
finer material to the Dothio. The mouth of the river threads
through a beach ridge plain to a small sandy delta, off which
there are shoals deposited from floodwaters in a sea that is
stained red by suspended clay in the nearshore zone. Much of the
impact of mining activities on the Dothio and the coastline
adjacent to its mouth derives from the lower catchment. At
present the river mouth is enclosed by a barrier spit that has
grown northward from the Thio delta and become attached to the
coast by deposition of sediment, mainly from the Dothio.

Nakéty

The lower part of the Nakéty catchment (area: 106 sq km)
consists of ultrabasic massifs subject to extensive opencast
nickel mining, and the delta has received an augmented yield of
red clay, sand, and fine gravel. Comparison of 1954 and 1976 air
photographs showed the growth of a littoral spit attached to the
beach of St. Paul and the formation of an alluvial island,
related to augmented sedimentation produced by mining activity
from the late 1950s to the 1970s. Subsequently the beach ridge
plain that borders the river mouth has been truncated by wave
erosion, undermining some of the coconut trees that grew on the
sandy terrain (plate 23). In general, beach erosion is unusual
adjacent to the mouths of New Caledonian rivers that have
sediment yields augmented by the effects of opencast mining, as
river floods have generally supplied sufficient sand and gravel
to maintain or prograde such beaches. It is possible that the
beach erosion behind the Baie de Nakéty is a temporary
phenomenon induced by the storm surge that accompanied Cyclone
Gyan in December 1981, and that in due course material from sandy
shoals off the river mouth will be carried shore-ward by wave
action to rebuild the eroded sector.

Canala

The Canala river system (catchment: 290 sq km) drains a
mountainous area north of the Plateau de Dogny, within which
Mesozoic sedimentary and volcanic rocks predominate. In the lower
catchment, above Boakaine, there are areas of opencast nickel
mining which have produced waste material spilling down slopes
and into lateral valleys. However, as only this part of the
drainage basin is in ultrabasic rocks, the effects on the Canala
River have so far been moderate in the lower reaches; moreover,
antipollution works have recently been introduced in the Boakaine
area. Some down-washed red clay has been deposited in the
extensive mangrove swamps that border the Canala delta, built
into the head of the Baie de Canala, but the mangroves remain
healthy and are advancing seawards as this accretion proceeds.
Between 1954 and 1976 an alluvial islet in one of the
distributaries grew from about 180 x 100 metres to about 260 x
120 metres as the result of mud accretion and mangrove spread.

Ouango

North-west of Canala, the Ouango drains a small but steep
catchment (area: 16 sq km) on the eastern side of the Me Aiu
Plateau. Opencast mining was extensive in the headwater region in
the 1960s and 1970s, and vast masses of sand and gravel have been
spread across the valley floor in a wide fan (plate 24) that
extends down to the delta, which is growing out into a branch of
the Baie de Canala. The delta consists of lobes of sand and
gravel bordered to seaward by wide red mudflats formed by the
accumulation of downwashed ferruginous clay (plate 25). The
effects of nickel mining on this valley and delta have been
severe.

Karoipa

The hilltop nickel mining that has discharged such large
masses of waste material into the Ouango valley has also affected
the Karoipa valley to the north. Although the catchment is small
(area: 23 sq km), the river channel has been much widened, and a
delta of sandy sediment, fringed seaward by red clay, is growing
into the southern part of the Baie de Kouaoua (plate 26).

Kouaoua

The Kouaoua river basin, with its large tributary, the
Kakenjou (combined catchment: 253 sq km), drains an area
dominated by the ultrabasic massifs of Me Aiu to the south and
Ménazi to the west. In the latter, opencast hilltop nickel
mining has been very extensive during the last 15 years. In 1981
the Ménazi massif was the chief mining centre in New Caledonia,
the SLN mine producing 779,000 tonnes of ore and Pentecost
Kouaoua a further 462,000 tonnes. In consequence, the Kouaoua and
especially the Kakenjou have received substantial amounts of
coarse sediment, their channels have widened, and deposition of
sand and gravel is building up spits and bars in the deltaic
region near Aoumou. The configuration of the coastline at the
mouth of the Kouaoua has changed considerably in recent years, as
shown on successive air photographs, and the present mouth is
almost enclosed by a long curving sand spit, built up by wave
action in the months since the floods generated by Cyclone Gyan
washed out the previous bordering spits from the river mouth.
Red-stained sea and offshore shoals in the Baie de Kouaoua off
the river mouth are further indications of changes due to the
impact of mining activity in the hinterland.

Koua

The Koua catchment (195 sq km) extends back to the crest of
the Central Ranges, consisting of Palaeozoic and Mesozoic
schists. The upper basin includes part of the peridotite Grand
Massif du Sud, relatively wide in this region. The nickel
deposits on the north slope of the Ménazi massif have been
intensively mined for two decades, but the mines are more than 15
km inland and mining waste from this source has not yet extended
far downstream. In the coastal regions there are areas of active
and historical mining north of the river at a low level, and
waste material from these has aggraded the channel, inducing bank
erosion and meander displacement. Arrival of additional sediment
at the river mouth has built up the delta, the bordering sandy
beach having been widened and extended along the whole length of
the delta shoreline between 1954 and 1976 as the result of
augmented accretion.

Baie de Poro

The coastal peridotite massifs have here been subject to
open-cast nickel mining by the Société Le Nickel. In 1981 this
region was the fourth largest mineral producer in New Caledonia,
but in 1982 mining ceased here. By then large quantities of red
clay, sand, and gravel had moved directly down coastal slopes to
the present shoreline. Much of the augmented sediment yield,
however, is travelling to the sea by way of the overloaded Goua
stream (catchment: 9 sq km), at the mouth of which sand and
gravel are being washed into the sea. To the north, pisolitic
ironstone sands and fine gravels have been carried down from
coastal slopes to form a depositional terrace prograded on to the
fringing coral reef that lines the coast up to Paraouyé (plate
27). Locally, the beach of dark sand has been cemented into
layers of beach rock by calcium carbonate precipitation. The
delivery of sand to this coast is evidently the outcome of
prospecting and mining activities in the coast range during the
past century, so that the beach rock must be of very recent
origin.

Houaïlou

The Houaïlou and its tributaries, notably the Neaoua, drain
an extensive basin (combined catchment: 539 sq km) north-east of
the Col des Roussettes in the Central Ranges. In its head-waters
the ultrabasic massif of Me Maoya has been subject to opencast
nickel mining, and some waste material has been carried
downstream. Torrential runoff during Cyclone Gyan in December
1981 widened the river channel and carried masses of sand and
gravel, including peridotite boulders, further downstream, with
sediment of diminishing calibre extending still further. At the
river mouth sandy material washed down during floods has been
built by wave action into a chain of spits and barrier islands.
In this deltaic region the Houaïlou is joined by the Néaoua
River from the south, draining valleys cut into ultrabasic
massifs that have been extensively mined for nickel. Red clay,
silt, sand, and gravel are being carried by this river down to
its confluence with the Houaïlou, so that coastal deposition has
been modified in recent decades by the effects of hinterland
mining activity. Between 1954 and 1976 the delta shoreline was
changed by the erosion of a wooded depositional island 250 metres
long and 30 to 50 metres wide, and by the redistribution of
derived sandy sediment by longshore drifting to prograde the
coast to the north-east.

Ho

West of the ultrabasic plateau that forms the peninsula of Cap
Bocage, the lower basin of the Ho River (catchment: 23 sq km)
contains some opencast nickel mines, and pisolitic ironstone
sands derived from the ferruginous lateritic crusts have been
carried downstream and built by wave action into beach ridges
bordering the river mouth in Baie Ugué. The effects of
hinterland mining activity are only slight at the river mouth at
this stage, but in the north-east of the Baie Ugué the fringing
coral reef has been severely damaged by accumulation of waste of
varying calibre from the Pounehoa mine on the Cap Bocage plateau,
swept directly down the steep coastal slope into the sea.

Néavin and Monéo

The Néavin and Monéo rivers, with a combined catchment of
202 sq km, mark the northern limit of the ultrabasic Grand Massif
du Sud on the east coast. The Néavin valley drains an area that
has received some mining waste from hilltop excavations to the
south, but the rest of the catchment is in strongly folded
Mesozoic sedimentary and volcanic rocks which have not been
mined. The impact of mining activities on this river system has
therefore been minor. At and below the confluence of the Néavin
and Monéo is a deltaic area with mangroves on shoals and a
seaward fringe of spits and beach ridges built of grey sand and
gravel, much of which has come from laterized ultrabasic
hillsides in the coastal fringe. As with the Thio, the Kouaoua,
and the Houailou, there have been marked changes in the last two
decades in the configuration of spits bordering the river mouth,
especially since these were washed through by floodwaters during
Cyclones Alison and Gyan.

The North-Eastern Rivers

North of the Monéo the rivers of north-eastern New Caledonia
drain catchments that do not contain large ultrabasic massifs and
have only small outliers of overthrust peridotite. In sequence,
with catchment areas in parentheses, the major rivers are the
Mou, or Oué Pouémaeu (58 sq km), the Nimbaye (321 sq km), the
Tchamba (187 sq km), the Amoa (182 sq km), the Hienghène (155 sq
km), and the Ouaième (338 sq km). In their middle and lower
reaches these rivers carry loads of generally grey sand and
gravel, but at their mouths they are typically estuarine inlets
or deep rias, in most cases with variable sand spits at the
coast. The air photograph taken in 1976 of the Ouaième ria shows
its mouth blocked by a sand barrier, but this was swept away by
floods following subsequent cyclones, especially Cyclone Gyan in
1981 (which raised the river level 14.4 metres at the head of the
inlet), and has subsequently been rebuilt as a cuspate foreland
south of the river mouth.

The small Massif de Grandié, on the watershed between the
Amoa and the Tiwaka, has not been subject to mining activity, and
these rivers have not been affected by downwashed waste. The
Massif de Tchingou, in the Tiwaka basin, has been prospected and
mined on a small scale at two locations, and the small amount of
mining waste has slightly affected some of the headwaters of this
river. The upper basin of the Tipindjé includes two small
peridotite overthrusts: the Oua Tilou massif, where mining
activity has been very slight, and the small Massif de Poindas,
subject recently to more intensive mining, which has so far led
to waste pollution only in a few head-streams and in the upper
reaches of the Tipindjé.

Conclusion

This review of the major river systems of the Grande Terre of
New Caledonia has indicated the extent of impact of opencast
mining on ultrabasic massifs in terms of changes on hillslopes,
in valley floors, along river channels, on deltas, and in
adjacent coastal areas. Figure 21 indicates the pattern of impact
at river mouths around New Caledonia. This impact has been severe
on the Oué Bouameu, the Tinip, the Poué Koué, the Népoui; the
Coulée, and the Rivière des Pirogues, on the west coast, and on
a number of east coast rivers, notably the Ouango, the Karoipa,
the Goua, and the Néaoua flowing into the Houailou. On a number
of other rivers, notably the Pouembout, the Ouha, the Moindah,
the Rivière du Cap, the Ouenghi, the Tontouta, the Dumbéa, and
on the east coast the Ouinné, the Ngoye, the Thio, the Nakéty,
the Canala, the Kouaoua, the Koua, and the Houaïlou, the effects
are so far slight or moderate but may become more severe as waste
from existing mines is added to the river system and as existing
augmented channel loads move downstream. It is evident that
opencast hilltop mining in the ultrabasic massifs of New
Caledonia has profoundly modified upland areas and has had a
spreading influence on the features of river channels, river
mouths, and adjacent coastal environments.

The discharge of gravels to the mouth of the Koumac River,
which drains a catchment unaffected by opencast mining, is an
indication that a coarse-grained sediment yield is not
necessarily the outcome of such mining. The rapid growth of the
Comboui delta, on the east coast, is partly due to ravine erosion
of hillslopes and partly to disturbance by mineral prospecting,
yielding eroded sand and silt to the river mouth. Arrival of red
clay and pisolitic ironstone sands at river mouths is probably
the most reliable indicator of modifications due to opencast
hilltop mining of the laterized mantles of ultrabasic massifs.
While the effects of such mining are prominent in the landscapes
of New Caledonia, it is likely that road-making and mineral
prospecting have greatly augmented fluvial sediment yields, both
in catchments affected by opencast mining and in those where no
mining has taken place. Other factors such as clearing, burning,
and grazing of the natural vegetation cover as well as
cultivation in hilly country have accelerated runoff and
increased the yields of sediment to New Caledonian rivers.
Mention has been made of the existence, particularly on hill
country inland from the central west coast, of abandoned
cultivation terraces (tarodières), built by indigenous people in
the pre-colonial era. The yields from such land management
systems would have been mainly fine-grained (sand, silt, and
clay), in contrast with the wider range of grain sizes (up to
gravel, cobbles, and boulders) in the waste materials generated
by opencast hilltop mining on deeply weathered ultrabasic
formations during the past century.